1. Field of the Invention
Electrolytic capacitors and, more particularly, a method for assembling multiple anode stacked capacitor configurations with a temporary adhesive, to aide in the alignment of separator materials and electrodes without sacrificing energy density, and electrolytic capacitors comprising such configurations.
2. Background of the Invention
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors typically consist of a cathode electrode, an electrically conductive electrolyte and a porous anode with a dielectric oxide film formed thereon. While aluminum is generally used for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. Flat constructions for aluminum electrolytic capacitors are known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween and connections between the various anode and cathode layers made via tabs on each individual electrode layer.
The need for high voltage, high energy density capacitors is most pronounced when employed in implantable cardiac defibrillators. Since the capacitance of an electrolytic capacitor is provided by the anodes, a clear strategy for increasing the energy density in the capacitor is to minimize the volume taken up by paper and cathode and maximize the number and volume of the anodes. For example, a multiple anode flat, stacked capacitor configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. In order to achieve higher energy densities, three, four and five anodes can be stacked per layer. Maximization of the anode volume may also be accomplished by etching to achieve more effective anode surface area, and making the relative size of the anode plates larger with respect to the cathode plates.
In optimizing flat stack capacitors, the tolerances of the individual components are necessarily tight. In order to achieve such tight tolerances but maintain physical separation between the anode and cathode electrodes during assembly, alignment features such as edge feet, notches, and holes are often placed on the active electrodes. These features perform their desired alignment functions adequately, but subtract material from the electrode that could be used to further optimize energy density. Alternate methods require tails on the electrodes that will subsequently be removed upon assembly into a capacitor case. These methods are sufficient for initial assembly, but do not guarantee that slippage will not occur once the smaller electrode is freed from the tail during the remaining assembly steps.
Once assembled, the tight compression of the case around the stack will prevent any further slippage. However, if an electrode is allowed to slip and become unaligned with respect to the separator and/or an alternate electrode during assembly, a short path can easily develop compromising the component's ability to store energy. Therefore, what is needed is an improved method of assuring alignment of separator materials and electrodes during the assembly of flat stack capacitors.